Loudspeakers, or speakers, are well known in the art and are commonly used in a variety of applications, such as in home theater stereo systems, car audio systems, indoor and outdoor concert halls, and the like. A loudspeaker typically includes an acoustic transducer comprised of an electro-mechanical device which converts an electrical signal into acoustical energy in the form of sound waves and an enclosure for directing the sound waves produced upon application of the electrical signal.
An example of structure in the conventional loudspeaker is shown in FIG. 1. The loudspeaker 11 includes a speaker cone 13 forming a diaphragm 17, a coil bobbin 25, and a dust cap 15. The diaphragm 17, the dust cap 15 and the coil bobbin 25 are attached to one another. The voice coil 27 is attached around the coil bobbin 25. The voice coil 27 is connected to suitable leads (not shown) to receive an electrical input signal through the electrical terminals.
The diaphragm 17 is provided with an upper half roll 21 at its peripheral made of flexible material. The diaphragm 17 connects to the speaker frame 19 at the upper half roll 21 by means of, for example, an adhesive. At about the middle of the speaker frame 19, the intersection of the diaphragm 17 and the coil bobbin 25 is connected to the speaker frame 19 through a spider (inner suspension) 23 made of a flexible material. The upper half roll 21 and the spider 23 allow the flexible vertical movements of the diaphragm 17 as well as limit or damp the amplitudes (movable distance in an axial direction) of the diaphragm 17 when it is vibrated in response to the electrical input signal.
An air gap 41 and annular members including a pole piece 37, a permanent magnet 33, and an upper (top) plate 35 make up a magnetic assembly. In this example, the pole piece 37 has a back plate 38 integrally formed at its bottom. The pole piece 37 has a central opening 40 formed by a pole portion 39 for dissipating heat generated by the voice coil 27. The permanent magnet 33 is disposed between the upper plate 35 and the back plate 38 of the pole piece 37. The upper plate 35 and the pole piece 37 are constructed from a material capable of carrying magnetic flux, such as steel. Therefore, a magnetic path is created through the pole piece 37, the upper plate 35, the permanent magnet 33 and the back plate 38 through which the magnetic flux runs.
The air gap 41 is created between the pole piece 37 and the upper plate 35 in which the voice coil 27 and the coil bobbin 25 are inserted in the manner shown in FIG. 1. Thus, when the electrical input signal is applied to the voice coil 27, the current flowing in the voice coil 27 and the magnetic flux (flux density) interact with one another. This interaction produces a force on the voice coil 27 which is proportional to the product of the current and the flux density. This force activates the reciprocal movement of the voice coil 27 on the coil bobbin 25, which vibrates the diaphragm 17, thereby producing the sound waves.
For a loudspeaker described above, heat within the loudspeaker and distortion of sound can be problematic. The voice coil is constructed of a conductive material having electrical resistance. As a consequence, when an electrical signal is supplied to the voice coil, the electric current flowing through the coil generates heat because of the interaction with the resistance. Therefore, the temperature within the loudspeaker and its enclosure will increase. A substantial portion of the electrical input power is converted into heat rather than into acoustic energy.
Such temperature rise in the voice coil creates various disadvantages. As an example of disadvantage, it has been found that significant temperature rise increases the resistance of the voice coil. This, in turn, results in a substantial portion of the input power of the loudspeaker to be converted to the heat, thereby lowering the efficiency and performance of the loudspeaker. In particular, it has been found that increased resistance of the voice coil in the loudspeaker can lead to non-linear loudness compression effects at high sound levels.
When additional power is supplied to compensate for the increased resistance, additional heat is produced, again causes an increase in the resistance of the voice coil. At some point, any additional power input will be converted mostly into heat rather than acoustic output. Further, significant temperature rise can melt bonding materials in the voice coil or overheat the voice coil, resulting in permanent structural damage to the loudspeaker.
Moreover, in the audio sound reproduction involving such a loudspeaker, it is required that the loudspeaker is capable of producing a high output power with low distortion in the sound waves. Low distortion translates to accurate reproduction of sound from the speaker. It is known in the art that a loudspeaker is more nonlinear and generates more distortion in lower frequencies which require large displacement of the diaphragm.
In order to solve this problem, it has been proposed to use a ring (cylinder) shaped conducting material (hereafter “shorting ring”) around a pole piece. The shorting ring stabilizes the magnetic field against changes caused by the current in the voice coil. The shoring ring acts as a short circuit winding that generates an inversely directed magnetic flux to counter the modulating effect of the voice coil on the flux in the permanent magnetic field. However, this arrangement does not, by itself, provide an efficient cooling mechanism. Thus, there is a demand for a loudspeaker that can dissipate heat efficiently while minimizing distortion of sound at the same time.